A semiconductor device having an IGBT is used in an electronic circuit for driving an inductive load such as a motor. A semiconductor device having a typical IGBT has the following structure.
An N−-type drift later is formed on a P+-type collector layer, a P-type base layer is formed in a surface portion of the N−-type drift layer, and an N+-type emitter layer is formed in a surface portion of the P-type base layer. Multiple trenches, which reach the N−-type drift layer by penetrating the P-type base layer and the N+-type emitter layer, are formed in a pattern of stripes. A gate insulation layer and a gate electrode are formed in order on a wall of each trench so that a trench gate can be formed with the trench, the gate insulation layer, and the gate electrode. Further, an emitter electrode is formed on the P-type base layer and the N+-type emitter layer through an interlayer dielectric film. The emitter electrode is electrically connected to the P-type base layer and the N+-type emitter layer through a contact hole of the interlayer dielectric film. Further, a collector electrode is formed on a back side of a collector layer and electrically connected to the collector.
In this type of semiconductor device, when a predetermined gate voltage is applied to the gate electrode, an N-type inversion layer is formed in the P-type base layer at an interface with the gate insulation layer in the trench, and an electron accumulation layer is formed in the N-type drift layer at an interface with the gate insulation layer in the trench. Electrons flow into the N−-type drift layer from the N+-type emitter layer through the inversion layer and the accumulation layer, and holes flow into the N−-type drift layer. Thus, a resistance decreases due to conductivity modulation so that it can change to an ON state.
In this type of semiconductor device having an IGBT, an ON voltage can be lowered compared to a semiconductor device having a MOSFET Recently, however, there has been a demand for a further reduction in an ON voltage.
For the above reason, for example, a patent document 1 discloses that a distance between adjacent gate electrodes is reduced to a very small value from 0.55 nm to 0.3 μm.
Further, a patent document 2 discloses that a trench gate has an enlarged portion which is located in an N−-type drift layer and has a width larger than a width of a portion other than the enlarged portion. Accordingly, a distance between the enlarged portions of adjacent trench gates is smaller than a distance between the other portions of adjacent trench gates.
In semiconductor devices as disclosed in the patent documents 1, 2, holes flowing into the N−-type drift layer are less likely to be drawn to the P-type base layer through space between adjacent trench gates, so that a lot of holes are accumulated in the N−-type drift layer. Thus, the amount of electrons flowing into the N−-type drift layer from the N+-type emitter layer through the inversion layer and the accumulation layer is increased. Since electron mobility is greater than hole mobility, an ON voltage is further reduced.
In the patent documents 1, 2, a low ON voltage is achieved. Recently, however, there has been a demand for a semiconductor device having not only a low ON voltage but also an improved load short-circuit tolerance.
That is, in such a semiconductor device, when a load short-circuit occurs, an electric current increases to saturation limited by the device. Then, Joule heat proportional to the salutation current is generated, so that a temperature of the semiconductor device increases. As a result, the semiconductor device may be broken.